1. Field of the Invention
In general, the present invention relates to a magnetic read head utilizing a magnetoresistive effect such as an AMR head or a spin-valve head. In particular, the present invention relates to a magnetoresistive head which sustains the linear response characteristic of the magnetoresistive effect, reduces the amount of Barkhausen noise, lessens the effect of problems encountered in the conventional antiferromagnetic film and effectively applies a bias generated by an exchange coupling magnetic field.
2. Description of the Related Art
Magnetic read heads utilizing a magnetoresistive effect of the conventional technology include an AMR (Anisotropic Magnetoresistance) head based on anisotropic magnetoresistive phenomena and a GMR (Giant Magnetoresistance) head based on spin scattering phenomena of conduction electrons. An example of the GMR head disclosed in U.S. Pat. No. 5,159,513 is a spin-valve head exhibiting a high magnetoresistive effect caused by a weak external magnetic field.
FIGS. 9 through 11 are diagrams showing a skeleton configuration of elements composing the AMR head according to the conventional technology. Reference numeral 1 shown in the figures is a soft magnetic film and reference numeral 2 is a nonmagnetic film. Reference numerals 3 and 4 are a magnetoresistive film and an antiferromagnetic film made of an FeMn alloy respectively. Reference numeral 5 is a ferromagnetic film whereas reference numeral 7 denotes a permanent magnetic film (a hard film). Reference numeral 8 is an antiferromagnetic film.
In order to operate a magnetoresistive head, two bias magnetic fields are required for the magnetoresistive film 3 which exhibits a magnetoresistive effect. One of the bias magnetic fields is used to make changes in resistance in the magnetoresistive film respond linearly to a magnetic flux from a magnetic recording medium. This bias magnetic field is applied in a Z direction perpendicular to the surface of the magnetic recording medium as shown in the figures and is called a lateral bias.
Normally called a longitudinal bias, the other bias magnetic field is applied in an X direction parallel to the surface of the magnetic recording medium and the magnetoresistive film 3. The longitudinal bias magnetic field is used for reducing the amount of Barkhausen noise which is generated by formation of a plurality of magnetic domains by the magnetoresistive film 3. In other word, the longitudinal bias magnetic field makes the change in resistance with the magnetic flux from the magnetic recording medium smooth. It is necessary to put the magnetoresistive film 3 in a single-domain state in order to reduce the amount of Barkhausen noise. There are two methods for applying the longitudinal bias for that purpose. According to one of the methods, the permanent magnetic films 7 are located at both the sides of the magnetoresistive film 3 and a leaking magnetic flux from the permanent magnetic films 7 is utilized as is shown in a structure of FIG. 10. According to the other method, on the other hand, an exchange coupling magnetic field developed on each of the contact boundary surfaces of the magnetoresistive film 3 and the antiferromagnetic films 8 is utilized as is shown in a structure of FIG. 11.
It is obvious from the structure shown in FIG. 11 wherein a bias magnetic field is generated from an exchange coupling magnetic field that this method is characterized in that the magnetoresistive film 3 is also created and extended at both ends beyond the region of the read track of the magnetic recording medium. The antiferromagnetic films 8 are created, coming in direct contact with the extended portions of the magnetoresistive film 3 to generate an exchange coupling magnetic field on each of the contact boundary surfaces between the magnetoresistive film 3 and the antiferromagnetic films 8. By pinning the direction of magnetization in the regions at both the ends of the magnetoresistive film 3 in the read-track direction (that is, the X direction shown in the figure), a bias for putting the magnetization of the read-track region of the magnetoresistive film 3 into a single-domain state in the X direction can be obtained.
The structure shown in FIG. 11 has the following problems. One of the problems is that, in spite of the fact that the magnetization in the magnetoresistive film 3 in each of the regions outside the read track is pinned in the X direction by the exchange coupling with the antiferromagnetic film 8, the direction of the magnetization in the magnetoresistive film 3 in the region outside the read track is changed by a magnetic flux from the magnetic recording medium in the Z direction shown in the figure because, normally, the intensity of the exchange coupling magnetic field is of the order in a range of several tens to 200 Oe. As a result, a magnetoresistive effect is observed also in each of the regions at both the ends in which region a magnetoresistive effect should never be observed. This problem gives rise to an inconvenience that the read track width can not be determined.
The other problem is that, since portions the magnetoresistive film in the regions at both the ends outside the read track are contiguous with the portions of the magnetoresistive film inside the read track, noise and irreversibility of the change in magnetization in the magnetoresistive film in the regions at both the ends outside the read track directly affect the change in magnetization of the magnetoresistive film inside the read track, giving rise to generation of Barkhausen noise and irreversibility of the change in magnetization in the magnetoresistive film inside the read track.
It is obvious from the structure shown in FIG. 10 wherein a bias magnetic field is generated by the permanent magnetic film that the permanent magnetic films 7 are located at both ends of the read-track region and that the direction of magnetization of each of the permanent magnetic films 7 is pinned in the read-track direction (that is, the X direction shown in the figure) by magnetic polarization. By applying a magnetic flux leaking from the permanent magnetic film 7 in the X direction into the magnetoresistive film 3, a bias for putting the magnetization of the magnetoresistive film 3 in a single-domain state in the read-track direction can be obtained.
The portions of the soft magnetic film 1, the nonmagnetic film 2 and the magnetoresistive film 3 at both the ends of the read track, which portions are in contact with the permanent magnetic films 7, must each be formed into a taper shape in order to stabilize the contact resistance against a current for detecting a magnetic resistance flowing from the permanent magnetic film 7 at one end to the soft magnetic film 1, then to the nonmagnetic film 2, then to the magnetoresistive film 3 and finally to the permanent magnetic film 7 at the other end. However, the taper shape gives rise to the following problems in the magnetic characteristics of the permanent magnetic film 7.
One of the problems is that the soft magnetic film 1, the nonmagnetic film 2 and the magnetoresistive film 3 each become an underlayer in the process of manufacturing the permanent magnetic film 7 at the tapered sections. In general, the magnetic characteristics of a permanent magnetic layer are affected very easily by the underlayer thereof. In the case of the structure shown in FIG. 10, the magnetic characteristics of the permanent magnetic film 7 in close proximity to the boundary surface facing the soft magnetic film 1, the nonmagnetic film 2 and the magnetoresistive film 3 are affected by the three underlayers of different types. As a result, it is extremely difficult to obtain stable magnetic characteristics.
The other problem is that, in order to put the magnetization of the magnetoresistive film 3 in a single-domain state in the read-track direction (that is, in the X direction shown in the figure), the permanent magnetic film 7 is polarized so as to orientate a number of magnetic components thereof in the read-track direction. None the less, since the coercive force of the permanent magnetic film 7 is of the order of several hundreds of Oe at the most, the direction of magnetization in the magnetoresistive film 3 can not be prevented from swinging subtly from the read-track direction due to the magnetic flux from the magnetic recording medium. That is to say, when the permanent magnetic film 7 is brought into direct contact with the magnetoresistive film 3, ferromagnetic coupling is developed between the permanent magnetic film 7 and the magnetoresistive film 3. As a result, fluctuations in magnetization occurring in the permanent magnetic film 7 directly affect the direction of magnetization in the magnetoresistive film 3.
If the fluctuation in magnetization occurring in the permanent magnetic film is smooth, the effect of the fluctuation on the magnetoresistive film is also smooth as well, giving rise to no problems. If the fluctuation is not smooth but irreversible instead or if Barkhausen noise is generated, on the other hand, there will be an irreversible effect on the change in response of the magnetoresistive film to the magnetic flux from the magnetic recording medium or there will be noise in the change in response, giving rise to generation of Barkhausen noise in the magnetoresistive film itself.
The structure shown in FIG. 9 is the structure of a conventional magnetoresistive head disclosed in Japanese Published Unexamined Patent Application No. Hei 7-57223 (1995). In this structure, a bias applied to the magnetoresistive film 3 for putting the magnetization of the magnetoresistive film 3 in a single-domain state in the X direction is obtained by applying a magnetic flux of the ferromagnetic layer 5 magnetized in the X direction by exchange coupling with the antiferromagnetic film 4 into the magnetoresistive film 3 and, at the same time, ferromagnetic coupling is developed on the contact boundary surface between the ferromagnetic film 5 and the magnetoresistive film 3.
The following problems are encountered in the structure shown in FIG. 9. The intensity of an exchange coupling magnetic field of the ferromagnetic film 5 experiencing exchange coupling with the antiferromagnetic film 4 is of the order of 50 Oe in the case of an NiFE ferromagnetic film 5 exchange-coupled with an FeMn antiferromagnetic film 4 with the film thickness of the former set at 300 xc3x85. In spite of the magnetization in the X direction by the exchange coupling, the direction of magnetization can not be prevented from fluctuating subtly due to the magnetic flux from the magnetic recording medium.
In the case of the exchange-coupled ferromagnetic film 5 brought into direct contact with the magnetoresistive film 3, ferromagnetic coupling is developed between the ferromagnetic film 5 and the magnetoresistive film 3. Thus, variations in magnetization occurring in the ferromagnetic film 5 directly affect variations in magnetization occurring in the magnetoresistive film 3. There is no guarantee at all that fluctuations in magnetization occurring in the exchange-coupled ferromagnetic film 5 which fluctuations are caused by the magnetic flux from the magnetic recording medium are smooth as is the case with the permanent magnetic film 7 shown in FIG. 10. As a result, noise is generated in variations in response of the magnetoresistive film 3 to the magnetic flux from the magnetic recording medium, giving rise to generation of Barkhausen noise.
In a sandwich structure of a free magnetic layer 9, a nonmagnetic intermediate layer 10 and a pinned magnetic layer 11 shown in FIGS. 12 and 13 for obtaining an optimum operation of a spin-valve head, on the other hand, it is necessary to apply a bias in the read-track direction (that is, in the X direction shown in the figures) to the free magnetic layer 9 in order to put the free magnetic layer 9 in a single-domain state and to magnetize the free magnetic layer 9 in the read-track direction as well as to apply a bias to the pinned magnetic layer 11 in the Z direction which is perpendicular to the direction of magnetization of the free magnetic layer 9 in order to put the pinned magnetic layer 11 in a single-domain state and to magnetize the pinned magnetic layer 11 in the Z direction. In this structure, a magnetic flux generated by the magnetic recording medium in the Z direction shown in the figures does not change the direction of magnetization in the pinned magnetic layer 11, but changes the direction of magnetization in the free magnetic layer 9 in the range of 90xc2x0xc2x1xcex8 relative to the direction of magnetization in the pinned magnetic layer 11, allowing a linear response characteristic of the magnetoresistive effect to be obtained.
In order to pin the direction of magnetization in the pinned magnetic layer 11 in the Z direction shown in the figures, a relatively strong bias magnetic field is required. The stronger the bias magnetic field, the better the pinning of the direction of magnetization. A bias magnetic field of at least 100 Oe is required in order to prevent the direction of magnetization from fluctuating due to the magnetic flux from the magnetic recording medium, thus, overcoming an antimagnetic field in the Z direction shown in the figures.
As a method of producing such a bias magnetic field, an exchange anisotropic magnetic field which is developed by bringing an antimagnetic layer 12 into contact with the pinned magnetic layer 11 is normally adopted.
The bias applied to the free magnetic layer 9 is used for assuring the linear response characteristic and for reducing the amount of Barkhausen noise generated by creation of a number of domains. Normally, the same methods as those for generating the longitudinal bias in an AMR head is adopted for generating the bias applied to the free magnetic layer 9. According to this method, permanent magnetic layers 13 are located at both ends of the free magnetic layer 9 and a leaking magnetic flux from each of the permanent magnetic layers 13 is utilized. As an alternative method, an exchange anisotropic magnetic field developed on the contact boundary surface with an antiferromagnetic layer 15 is utilized.
As described above, by utilizing an exchange anisotropic magnetic field developed on a contact boundary surface with the antiferromagnetic layer for generating the longitudinal bias of an AMR head, and the bias of the pinned magnetic layer and the bias of the free magnetic layer of a spin-valve head, a magnetoresistive head can be implemented in which the linear response characteristic can be improved and the amount of Barkhausen noise can be reduced.
The exchange anisotropic magnetic field is a phenomenon caused by an exchange interaction among magnetic momentums on the contact boundary surface between the ferromagnetic film and the antiferromagnetic film. In the case of a ferromagnetic film made of an NiFe alloy, the antiferromagnetic film that generates an exchange anisotropic magnetic field in conjunction with the NiFe film is typically made of an FeMn alloy. However, the corrosion resistance of an FeMn film is very poor, giving rise to a problem that the exchange anisotropic magnetic field degrades substantially because development of corrosion has been under way during the manufacturing process and the operation of the magnetic head in addition to a problem that the magnetic recording medium is damaged. In addition, the temperature of regions in close proximity to the FeMn layer rises to about 120xc2x0 C. due to heat generated by a detection current during the operation of the magnetic head and the exchange anisotropic magnetic field generated by the FeMn film is sensitive to changes in temperature as is widely known. The intensity of the exchange anisotropic magnetic field are but linearly decreases with the increase in temperature to about 150xc2x0 C. known as a blocking temperature Tb at which the exchange anisotropic magnetic field disappears. As a result, a stable exchange anisotropic magnetic field can not be obtained.
An NiMn alloy or an NiMnCr alloy which has a face-centered tetragonal structure disclosed in U.S. Pat. No. 5,315,468 is known as an invention for improving the corrosion resistance and the blocking temperature of the FeMn film. Even though the corrosion resistance of the NiMn film is better than that of the FeMn film, it is still not sufficient for practical use. The NiMnCr film is made of NiMn doped with Cr in order to improve the corrosion resistance of the NiMn film. However, the improvement of the corrosion resistance by the Cr doping gives rise to a problem that the intensity of the exchange anisotropic magnetic field and the blocking temperature decrease.
In addition, in order to obtain an exchange anisotropic magnetic field from the NiMn or NiMnCr alloy, it is necessary to create an ordered-structure crystal of the CuAgxe2x80x94I type having a face-centered tetragonal structure on a portion of the antiferromagnetic film and, on the top of that, control of ordered-to-random transition and control of the volume density of ordered and random phases are required as a matter of course. As a result, in order to obtain stable characteristics, the process control and monitoring during the process of manufacturing the magnetic head can not help becoming very complex. There are also manufacturing-process problems in order to obtain the required exchange anisotropic magnetic field that magnetic-field heat treatment must be repeated a plurality of times and that the rate of decrease in temperature is low, taking a long time to decrease the temperature from a high value to a low one. For example, it takes 17 hours to have the temperature decrease from 255 xc2x0 to 45xc2x0 C. For more information, refer to Appl. Phys. Lett. 65(9), 29 Aug. 1994.
A method for creating a layer made of an Nixe2x80x94Fexe2x80x94Mn three-element alloy on an NiFe/FeMn boundary surface through diffusion by heat treatment carried out on an NiFe/FeMn stacked film at the temperature range 260xc2x0 to 350xc2x0 C. for 20 to 50 hours is disclosed in U.S. Pat. No. 5,809,109 as an invention for improving the blocking temperature of a film made of an FeMn alloy. The fact that this method is not effective for the improvement of the corrosion resistance, a big problem encountered in the film made of an FeMn alloy, can be understood with ease. On the top of that, the fact that the heat treatment requires a long time ranging from 20 to 50 hours gives rise to a problem in the manufacturing process.
According to publications such as xe2x80x9cMagnetic Material Handbookxe2x80x9d published by Asakura Shoten, a publisher. Mn-family alloys such as NiMn, PdMn, AuMn, PtMn and RhMn can be used as a ferromagnetic material. None the less, there is no comment with respect to the exchange anisotropic magnetic field on the contact boundary surface with the ferromagnetic film. On the top of that, there is no clear description at all regarding the characteristics of the ferromagnetic film itself and the exchange anisotropic magnetic field in the case of a super-thin film with a thickness of several hundreds of xc3x85.
It is a first object of the present invention to provide a structure of a magnetoresistive head which structure allows a stable bias magnetic field to be generated in a magnetoresistive film thereof in order to solve the aforementioned problems of the exchange coupling bias in the conventional structure shown in FIG. 11, overcome the aforementioned problems of the permanent magnetic bias in the conventional structure shown in FIG. 10, and eliminate the aforementioned problems of the bias in the structure of FIG. 9 disclosed to the public.
It is a second object of the present invention to provide a magnetoresistive (MR) head having an excellent linear response characteristic and a reduced amount of Barkhausen noise by providing an antiferromagnetic film which has excellent corrosion resistance and can apply a sufficient required exchange anisotropic magnetic field in the case of a super-thin film.
It is a third object of the present invention to provide an MR head having an excellent linear response characteristic and a reduced amount of Barkhausen noise by providing an antiferromagnetic film which has moderate dependence of an exchange anisotropic magnetic field on the temperature and a high blocking temperature.
It is a fourth object of the present invention to provide an MR head having an excellent linear response characteristic and a reduced amount of Barkhausen noise by providing an antiferromagnetic film which allows a heat treatment step for obtaining the characteristics described above to be implemented at a temperature and a rate of decrease in temperature and in a time which temperature, rate of decrease in temperature and time are applicable to a process of manufacturing an ordinary MR head.
The present invention provides a magnetoresistive head in which: ferromagnetic films exhibiting a magnetoresistive effect are used; in a read-track region at the center of the magnetoresistive head, a magnetoresistive film is created; at each of both ends of the magnetoresistive film outside the read-track region, an antiferromagnetic film and the ferromagnetic film experiencing an exchange coupling magnetic field due to direct contact with the antiferromagnetic film are created in such a way that the ferromagnetic film is not brought into direct contact with the magnetoresistive film so as to prevent ferromagnetic coupling from being developed by a contact boundary surface between the magnetoresistive film and the ferromagnetic film; and bias magnetization is applied to the magnetoresistive film by exchange coupling between the ferromagnetic film and the antiferromagnetic film.
In order to prevent the ferromagnetic film from being brought into direct contact with the magnetoresistive film, a film made of Ta is introduced between the ferromagnetic film and the magnetoresistive film as an intermediate layer or the ferromagnetic and antiferromagnetic films are created into a structure wherein the antiferromagnetic film is brought into direct contact with the magnetoresistive film.
In the magnetoresistive head provided by the present invention, a sufficient required bias magnetic field is applied by the antiferromagnetic film in direct contact with the ferromagnetic film exhibiting a magnetoresistive effect in order to make the response of the magnetoresistive effect to a magnetic flux from a magnetic recording medium linear and to reduce the amount of Barkhausen noise. The antiferromagnetic film is made of a PtMn alloy, heat treatment is carried out at temperatures in the range 200xc2x0 to 350xc2x0 C. after the ferromagnetic film in direct contact with the PtMn antiferromagnetic film is created; and a predetermined interdiffusion layer is created on the boundary surface between the PtMn antiferromagnetic film and the ferromagnetic film in direct contact with the PtMn antiferromagnetic film in order to generate an exchange anisotropic magnetic field.
The above heat treatment can be implemented at a temperature and a rate of decrease in temperature and in a time which temperature, rate of decrease in temperature and time are equivalent to those used in the process of manufacturing an ordinary magnetoresistive head, thus being a very practical heat-treatment method.
In addition, the corrosion resistance of the PtMn alloy is extremely excellent in comparison with those of the FeMn, NiMn and NiMnCr alloys. On the top of that, no corrosion is observed at all in a variety of solvents and cleaners during the process of manufacturing the magnetoresistive head. By the same token, the operation of the magnetoresistive head in a harsh environment is chemically stable.
In addition, the characterizing features of the antiferromagnetic film made of a PtMn alloy are that the exchange anisotropic magnetic field obtained by creating a predetermined interdiffusion layer on the boundary surface between the ferromagnetic film and the antiferromagnetic film made of an PtMn alloy in direct contact with the ferromagnetic film is very stable thermally in comparison with an exchange anisotropic magnetic field created by an FeMn antiferromagnetic film, and that the bias magnetic field is very stable in the range of the head operating temperature because it is possible to generate an exchange anisotropic magnetic field having a constant intensity in the range from the ambient temperature to 120xc2x0 C., the operating temperatures of the magnetoresistive head. By the same token, the blocking temperature of the PtMn alloy at which the exchange anisotropic magnetic field disappears is 380xc2x0 C. which is much higher than 150xc2x0 C., the blocking temperature of the FeMn alloy. As a result, the exchange anisotropic magnetic field is extremely stable during the process of manufacturing the magnetoresistive head and during the operation of the head.
On the top of that, in the case of the PtMn alloy, since it is possible to generate an exchange anisotropic magnetic field on a boundary surface either above or beneath of the ferromagnetic film in direct contact with the antiferromagnetic film made of an PtMn alloy, the exchange anisotropic magnetic field can be obtained without an underlayer film such as a film made of Ta for making the crystal orientations uniform as is required in order to obtain an exchange anisotropic magnetic field by using a film made of an FeMn alloy. As a result, it is now possible to build a device structure which can not be constructed so far due to restrictions imposed by the method of using the conventional antiferromagnetic film.